Neutral hexacoordinate silicon complexes

VI. NEUTRAL HEXACOORDINATE SILICON COMPLEXES A. Intramolecular Coordination... 

Relatively few ligand types have been used for the formation of neutral hexacoordinate silicon complexes, resulting in several complex types 182-193208 -218. Acetylacetonato (acac) chelates [182, (acac SiXY] were prepared directly from the reaction of... 

A novel (3,3) sigmatropic rearrangement of a hexacoordinate allyl-silicon complex (neutral tetraoxyspirosilicate) to a pentacoordinate complex was recently described242. The allyl group migrates from silicon to the a-carbon of a tropolone ligand242. 

Picoline was found to react in chloroform solution with bis(dichlorosilyl)amine to form two hexacoordinate silicon complexes the ionic [H2Si(3-MeC5H4N)4]2+-2Cl and a neutral H2Si(3-MeCsH4N)2Cl2. These complexes are in equilibrium with each other in chloroform solution. The crystal structure of the ionic complex was reported247. 

A subgroup of the neutral hexacoordinate silicon complexes discussed above is the family of binuclear complexes (55a,c,g,j). 

Summary The first ionic dissociation of the Si-Cl bond in neutral hexacoordinate silicon complexes is reported. An equilibrium reaction between the ionic siliconium chloride and its neutral precursor (dissociation-recombination) is observed. The population ratio can be controlled by temperature or by replacement of the chloro ligand by a triflate group. The reaction enthalpy and entropy of the dissociation are both negative, suggesting that solvent organization facilitates dissociation at low temperature. 

A convenient method for the preparation of neutral bis(N->Si) hexacoordinate silicon complexes has been developed and reported recently, consisting of ligand exchange between a polychlorosilane (1) and 0-trimethylsilyl derivatives of hydrazides (2, Eq. 1) [2]. An attempt to utilize this synthetic route for the preparation of isomeric 0->Si coordinated chelates did not lead to the expected hexacoordinate complexes, but to ionic siliconium chloride salts stabilized by two (O—>Si) dative bonds (5, Eq. 2) [3]. 

The spectral changes are completely reversible with temperature changes, and thus represent an equilibrium dissociation—recombination process, the first reported dissociation of a neutral hexacoordinate silicon complex [5], The Si chemical shifts of the equilibrium mixtures at various temperatures are listed in Table 1. 

The main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

The most widely studied group of hydrazide-based silicon complexes is the group of neutral hexacoordinate bis-chelates, with coordination of two dimethylamino donors to silicon. 

Si NMR Chemical Shifts of Neutral Hexacoordinate Bis-(N Si) Silicon Complexes with the N-NMe2 Donor Group [CDC13, 300 K, ( /, Hz)]... 

Molecular and Electronic Structure of Penta- and Hexacoordinate Silicon Compounds TaHe 18. Si Chemical Shifts (ppm) of Some Neutral Penta-coordinate Silicon Complexes... 

Recent studies on hydrazide-based hypercoordinate silicon complexes demonstrated the unusual flexibility of these compounds their tendency to reversibly transform between penta- and hexacoordinate compounds [1, 2] on the one hand and to irreversibly rearrange to more stable complexes [3] on the other. Thus, neutral hexacoordinate bis-chelate complexes (1) undergo reversible ionization in solution (Eq. 1), which is strongly dependent on a variety of factors temperature (ionization is enhanced at low temperatures), solvent (ionization takes place in hydrogen-bond donor solvents such as CHCI3, CH2CI2, and CHFCI2), the nature of the anion, the... 

Synthesis and Structural Characterization of Novel Neutral Hexacoordinate Silicon(IV) Complexes with 5 i02N4 Skeletons... 

Summary Treatment of Si(NCO)4 or Si(NCS)4 with 4-aminopent-3-en-2-ones yielded novel neutral hexacoordinate silicon(IV) complexes with an S/O2N4 framework, compounds 3-6. These silicon(IV) complexes were characterized in the solid state by single-crystal X-ray diffraction and Si VACP/MAS NMR spectroscopy. Compounds 3-5 crystallized as the (OC-6-12)-isomer, and 6 was isolated as the rranj-isomer. 

Scheme 1. Syntheses of the neutral hexacoordinate silicon(IV) complexes 3-6.

In contrast to the well-established chemistry of anionic and cationic hexacoor-dinate silicon(IV) complexes with SiOe skeletons, neutral hexacoordinate silicon(IV) species with SiOe frameworks are significantly less well explored. Compounds and 60 are examples of this type of compound. 

We have recently reported the first observation and measurement of spin-spin interactions in neutral hexacoordinate silicon chelates [1] which extend across the N— Si coordinative bond and over two, three, and even four bonds. These coupling constants were highly sensitive to small geometrical modifications in the complex. The major geometrical requirement found for spin-spin coupling over two bonds (N—>Si-F or N—>Si-H) in hexacoordinate complexes la-lc was that the corresponding bond angle be 90° or very close to it. 

The general synthetic method for the preparation of penta- [4] and hexacoordinate [5] neutral silicon complexes from 0-silylated hydrazides 2 has been described previously. The syntheses used in the present study are shown in Scheme 1. In both reactions the pentacoordinate complex is an intermediate on the way to the corresponding hexacoordinate species. However, the equilibrium condition is sufficiently favorable to allow the isolation of 4 as the major product simply by using an excess of 3. By contrast, the intermediate 6 is unstable, and is transformed spontaneously in solution at 300 K within 2-3 hours to 7. At lower temperatures the solution NMR spectra of 6 could be studied for several hours before it completely transformed to 7. 

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